The flexibility and reliability of communication networks based upon transmissions of light signals via optical fibers have been significantly increased by the availability of assemblies such as optical circulators and isolators. For example, a three-port circulator may be used to enable a single fiber to be used for bidirectional communications between two remote sites. By utilizing non-reciprocal optical elements, i.e. elements which affect light moving in different directions differently, a bidirectional fiber may be optically coupled to both an input fiber and an output fiber. Non-reciprocal operations provide differences in "walk-off," i.e. spatial displacement, of oppositely directed light beams, so that the input and output fibers are optically isolated from each other.
An optical isolator may include only single-mode fibers. An input fiber directs light signals into an optical assembly that splits the light into polarization components, performs non-reciprocal operations on the components, and recombines the components for output at an output fiber. The non-reciprocal operations are designed to reduce the likelihood that back-directed light will be aligned with the input fiber.
Systems that include optical circulators or isolators often have two or more fibers in a parallel relationship at one end of an assembly of optical elements which manipulate the polarization components of beams propagating through the assembly to or from one of the parallel fibers. Typically, each element in such an assembly is a unitary member within the path of all polarization components, even within systems designed to handle two parallel beams differently. This may require a greater number of optical elements within the assembly, thereby increasing signal crosstalk among fibers. Crosstalk within an optical circulator or isolator is the transmission of light from an input fiber to a fiber which is not the intended output fiber. One cause of signal crosstalk is back-reflection from various optical elements within the assembly.
As an alternative to the unitary optical elements, some optical circulators and isolators include divided optical elements. U.S. Pat. No. 5,204,771 to Koga describes an optical circulator having a divided optical element. One portion of the optical element is a reciprocal clockwise rotator, and the adjacent portion is a reciprocal counterclockwise rotator. Incoming light is separated into two components by a double refraction crystal plate that immediately precedes the divided element. The crystal plate directs one light component into the reciprocal clockwise rotator and the other light component into the reciprocal counterclockwise rotator. This allows the two light components to be separately but simultaneously manipulated. The Koga patent includes embodiments in which a single optical element has four portions that provide selective light component rotation. U.S. Pat. No. 5,471,340 to Cheng et al. also describes an optical assembly having a divided component-rotation element. The divided element is similar to the elements of Koga, since it is designed to achieve independent polarization rotation and includes rectangular element portions.
U.S. patent application Ser. No. 08/805,001 to Chang, filed Feb. 25, 1997, and assigned to the assignee of the present invention, also describes a split polarization rotator. A Faraday rotator is at a rearward side of the split polarization rotator. An incoming beam is divided into first and second polarization components. One polarization component passes through the "positive half" of the split polarization rotator and is rotated 45.degree. counterclockwise, but then is returned to its original orientation by a 45.degree. clockwise rotation induced by the Faraday rotator. On the other hand, the second polarization component passes through the "negative half" of the split polarization rotator, which causes a 45.degree. clockwise rotation prior to a second 45.degree. clockwise rotation by the Faraday rotator. Thus, the first polarization component is rotated a total of 0.degree. and the second polarization component is rotated by a total of 90.degree.. This Chang reference describes a method of fabricating the split polarization component. A pair of half-wave plate blocks having the desired optical properties for component rotation is provided. Each of the blocks is divided equally in a lengthwise dimension. A half from one of the blocks is coated with a thin layer of a suitable adhesive and brought into contact with a half from the other block. The adhesive is cured to form the split polarization rotator. A second split polarization rotator may be formed using the remaining two halves of the two blocks. While the method of manufacture operates well for its intended purpose, the process is labor intensive.
What is needed is a cost-efficient method of fabricating split optical elements which allow polarization components to be independently manipulated. Also needed is a split walk-off element that facilitates independent spatial separations of the polarization components.